The invention relates generally to communication systems and, more particularly, to decoding techniques for use in such systems.
Broadband wireless communication systems such as fixed wireless local loop and mobile satellite systems have been experiencing tremendous growth. These systems are mainly subject to slowly-varying Rician fading channels. As an example, consider a high bit-rate Time Division Multiple Access (TDMA) fixed wireless local loop system in which communication between base stations and stationary terminals occurs in bursts. For such a system, the channel is typically a slowly-varying, frequency-selective fading channel and can be considered constant over the duration of a packet or a time slot in a frame. Hence the channel can be viewed as a Gaussian Inter Symbol Interference (ISI) channel in each frame.
Depending on the channel and interference conditions, the modulation technique used in these systems can be, e.g., Quadrature Phase Shift Keying (QPSK), 8-PSK, 16-QAM (Quadrature Amplitude Modulation), 32-QAM or an even larger constellation. This allows the use of bandwidth efficient Trellis Coded Modulation (TCM), as described in, e.g., G. Ungerboeck, xe2x80x9cTrellis-Coded Modulation with Redundant Signal Sets, Part I. Introduction,xe2x80x9d IEEE Communications Magazine, Vol. 25, No. 2, pp. 4-11, February 1987, and G. Ungerboeck, xe2x80x9cTrellis-Coded Modulation with Redundant Signal Sets, Part II: State of the Art,xe2x80x9d IEEE Communications Magazine, Vol. 25, No. 2, pp. 12-21, February 1987. At high symbol rates, the delay spread can introduce ISI among many symbols and thus preclude the use of Maximum Likelihood Sequence Estimation (MLSE) type of equalizers for large QAM constellations. A Decision Feedback Equalizer (DFE) is therefore typically used to combat ISI of the transmitted symbols.
It is well known that the performance of a system using separate DFE and channel decoding is significantly degraded by error propagation in the DFE. See M. V. Eyuboglu and S. U. Qureshi, xe2x80x9cReduced-state Sequence Estimation for Coded Modulation on Intersymbol Interference Channels,xe2x80x9d IEEE Journal on Selected Areas in Communications, 7(6):989-995, August 1989. Several joint equalization and decoding techniques have been proposed to improve the performance of coded modulation techniques when a DFE is used. These techniques provide a variety of performance versus complexity tradeoffs.
One such technique, referred to as Parallel Decision-Feedback Decoding (PDFD), is described in, e.g., M. V. Eyuboglu and S. U. Qureshi, xe2x80x9cReduced-state Sequence Estimation for Coded Modulation on Intersymbol Interference Channels,xe2x80x9d IEEE Journal on Selected Areas in Communications, 7(6):989-995, August 1989, K. Wesolowski, xe2x80x9cEfficient digital receiver structure for trellis-coded signals transmitted through channels with intersymbol interference,xe2x80x9d Electron. Letters, pp. 1265-1267, November 1987, and A. Duel-Hallen and C. Heegard, xe2x80x9cDelayed decision-feedback sequence estimation,xe2x80x9d IEEE Trans. on Communications, Vol. 37, pp. 428-436, May 1989. PDFD is designed to reduce the complexity of an optimum joint MLSE technique. Even though PDFD is suboptimal in performance compared to the optimum joint MLSE technique, depending on the channel conditions, it can still achieve significant performance gains. The decoder trellis complexity of PDFD is the same as that of the corresponding TCM decoder trellis complexity, while the complexity of the optimum joint MLSE technique can be very high and therefore prohibitive for high bit-rate applications.
Recently, an alternative reduced-complexity equalization technique based on soft-decision feedback equalization (S-DFE) has been demonstrated to have a significant gain over traditional hard-decision feedback equalization (H-DFE) for QPSK modulation. See S. Aryavisitakul and G. Durant, xe2x80x9cA broadband wireless packet technique based on coding, diversity, and equalization,xe2x80x9d IEEE Communications Magazine, pp. 110-115, July 1998, S. Aryavisitakul and Y. Li, xe2x80x9cJoint coding and decision feedback equalization for broadband wireless channels,xe2x80x9d IEEE Journal on Selected Areas and Communications, pp. 1670-1678, December 1998, and S. H. Muller et al., xe2x80x9cReduced-state soft-output trellis-equalization incorporating soft feedback,xe2x80x9d IEEE GLOBECOM""96, pp. 95-100, November 1996. However, the gains obtained using S-DFE for large constellations are much smaller than what is obtained for QPSK. More particularly, as the constellation size increases, the magnitude of the error made by a S-DFE for an incorrect decision approaches that of a H-DFE. Hence the effect of error propagation in S-DFE is comparable to that of H-DFE and thus the gain is not that significant.
As is apparent from the above, a need exists for improved reduced-complexity decoding techniques which are suitable for use with large modulation constellations, and which overcome the problems associated with the conventional PDFD, S-DFE and H-DFE techniques.
The present invention provides improved Parallel Decision-Feedback Decoding (PDFD) techniques in which Decision Feedback Equalization (DFE) and channel decoding are performed jointly, and the DFE is implemented for multiple survivor paths, rather than for a single survivor path as in a conventional arrangement. The invention is suitable for use in decoding applications in which a sequence of symbols are decoded by determining a most-likely path through a finite state machine representation known as a trellis. In accordance with the invention, a DFE feedback path is computed separately for each survivor path of each state in the trellis. This is done at each decoding stage, i.e., for each stage t of the trellis. For a given stage t of the trellis, branch metrics are computed for each of the states, using a different input value yt(s) for each state. The input values yt(s) correspond to extended survivor paths computed for each state using the DFE feedback paths. The branch metrics are utilized to determine a global most-likely path through the trellis, and the most-likely path is used to generate a decoded output signal representative of the transmitted symbol sequence. This PDFD technique in accordance with the invention thus generates DFE values for each survivor path corresponding to each state in the trellis. The feedback path of each DFE uses, e.g., the last Nb decoded symbols in the survivor path of that state, where Nb is the number of taps in the DFE feedback path.
The invention allows PDFD to be applied efficiently in high bit-rate systems such as, e.g., broadband wireless local loop systems with stationary terminals. The invention can be utilized with a wide variety of different modulation constellations, such as, e.g., Quadrature Phase Shift Keying (QPSK), 8-PSK, 16-QAM (Quadrature Amplitude Modulation), 32-QAM or larger constellations. Advantageously, the joint DFE and decoding techniques of the invention achieve significant performance gains as compared to separate DFE and channel decoding techniques, for both PSK and QAM constellations.